1. Field of the Invention
The present invention relates to a rotation sensor mounted to a rotor for detecting a rotation angle of the rotor.
2. Description of the Related Art
For example, when detecting the rotation angle of a handle attached to a rotational shaft, such as a steering shaft of a motor vehicle, integrally therewith, so-called a rotation sensor is used (For example, see JP-A-2002-98506).
As an example of such a rotation sensor, there is one having stationary cores arranged so as to oppose to a rotor at a predetermined distance. An example of the structure of the rotation sensor relating to the description below will be described as a related technology. Since the structure of the related technology introduced here is common to a rotation sensor according to embodiments of the present invention in detailed structure except for the shape of a coil core or the shape of a sensing unit, which are specific for the present invention, it will not be shown in the drawings.
The rotation sensor includes a rotor mounted to a rotating shaft, stationary cores each having a core body formed of insulative magnetic material and at least one exciting coil stored in the core body, and a rotation angle detecting unit. The exciting coil includes, for example, four pairs of exciting coils, which are arranged at regular intervals in the circumferential direction of the rotor, respectively. Accordingly, the rotational angle between 0° and 360° of the rotor is detected.
The rotor and the stationary cores are attached to a fixed member positioned in the vicinity of a shaft, and are stored in a case formed of metal or insulative magnetic material having a shielding property with respect to an alternating magnetic field, respectively. The rotation sensor is adapted to detect the rotation angle of the shaft based on variation in impedance of the exciting coil caused by the rotation of the shaft.
The rotor includes a supporting member formed into a disk shape of resin material having a good sliding property, such as POM (polyoxymethylene), and a conductive sensing unit connected to the supporting member via a stay member and continuously varying in width circumferentially thereof. The sensing unit is formed of conductive metal having a narrow portion having the minimum width and a wide portion having the maximum width located on the radially opposite side of the narrow portion, and is formed so that the width in the radial direction of the sensing unit varies according to the rotation angle of the rotor, whereby an eddy current having a magnitude corresponding to the width in association with the rotation is induced by the alternating magnetic field. The impedance of the exciting coil varies with variations in the amount of the eddy current induced by the sensing unit. The sensing unit has a shape of simple outline in which the width is varied in the circumferential direction thereof by setting the centers of an inner diameter circle and an outer diameter circle at the positions so as to be deviated from each other for the sake of ease of formation thereof as described later (See FIG. 16).
One of the pair of stationary cores is mounted on a printed board constituting a measuring apparatus and is arranged so as to oppose to the other stationary core mounted to a case on the opposite side of the sensing unit of the rotor at a predetermined distance. The stationary core includes a core body formed of insulative magnetic material, and an exciting coil to be stored in the core body (See FIG. 17). Then, the predetermined exciting coils are connected in series, respectively, and establish a magnetic circuit around the stationary core by an alternating exciting current from a measuring unit.
The measuring apparatus includes a phase shifting unit, a phase shifting amount detecting unit, and a converter connected in four rows between a frequency divider circuit and a measuring unit. Output signals from a pair of converters are differentiated and then amplified by a differential amplifier, and are fed to an A/D converter of the measuring unit as output of the voltage signal. A shift level adjusting unit for adjusting the voltage level of the amplified voltage value is also connected to the differential amplifier.
An oscillation circuit outputs oscillation signals of a specific frequency to the phase shifting unit including a resistance, an exciting coil, and a capacitor via the frequency divider circuit. At this time, the phase of the voltage signals at both ends of the respective capacitors varies with variations in impedance, described above, of the respective exciting coils. The voltage signals at both ends of the capacitor are outputted to the phase shifting amount detecting unit. The respective phase shifting amount detecting unit detects the phase sifting amount of the voltage signals at both ends of the respective capacitors. The converter has a function to covert the detected phase shifting amounts into the corresponding voltage values.
The output, which corresponds to the amplified two differential signals and the output signals from the four converters are supplied to the measuring unit. Accordingly, the measuring unit compares the relative magnitudes of the four output signal levels in a first place. As a consequence, the positions where the respective exciting coils are disposed are determined by the sensing unit of the rotor. As shown in FIG. 15, an output according to the rotation angle detected by the four pairs of coils is obtained. As shown in the same drawings, the respective output voltages (V) of the exciting coils exhibit projected peaks at positions apart from each other by 180°, which are generated corresponding to the two connecting portions of the rotor.
In the same drawing, the rotation angle of the rotor is measured over the entire range between 0° and 360° using alternately repeated four areas A except for the projected peaks.
The rotor and the stationary core are mounted to the fixed member positioned in the vicinity of the shaft, and are stored in the case having an upper case and a lower case. The rotation sensor is adapted to detect the rotation angle of the shaft based on variations in the impedance of the exciting coils caused by the rotation of the rotor.
Subsequently, a problem in improvement of the detecting accuracy of the rotation sensor will be described. More specifically, problems occurring when assembling the rotation sensor and problems occurring when using the rotation sensor are described, respectively.
The problems occurring when assembling the rotation sensor will be described first.
FIG. 17 shows an appearance of a stationary core 700 used for the rotation sensor according to the related technology (See FIG. 17A), and a state of a magnetic flux generated in the stationary core (See FIG. 17B).
As is clear from FIG. 17A, a core body 720 of the stationary core 700 includes a disk-shaped base member 721, a core peripheral wall portion 722 projecting upward from the entire periphery of the base member 721, and a column shaped projection 723 projecting upward from the center of the base member 721. Accordingly, the magnetic flux generated in the stationary core 700 generates radially from the column shaped projection 723 of the core body surrounded by an exciting coil 710 (See an arrow in FIG. 17B). In other words, the magnetic flux is generated not only in the direction in which the sensing unit of the rotor extends, but also in the direction intersecting therewith.
In this manner, when the magnetic flux is generated in the direction intersecting with the sensing unit, and the relative position between the exciting coil and the core body is displaced or the center of the rotor and the center of the stationary core is displaced, variations occurs in the amount of magnetic flux of the stationary cores passing across the sensing unit of the rotor even when the displacement is within the tolerance in dimension or assembly of the components of the sensing unit or the stationary cores. Consequently, deviations in output properties may occur unless the tolerance level in dimension or assembly of the components of the sensing unit or the stationary cores is set up very small. As a consequence of containing such an error, in the case of the rotation sensor in the related technology, it is necessary to set the tolerance in dimension of all components or the tolerance in assembly of the components of the rotation sensor to an extremely small value when mass-producing the same.
Subsequently, the problems which occur when using the rotation sensor will be described. As described above, the sensing unit is designed so that the centers of two circles being different in size are slightly deviated from each other and, in this state, these two circles define an outline thereof in the sake of ease of design or manufacturing. More specifically, as an example, the respective centers of a circle of about 52 mm in diameter and a circle of about 57 mm in diameter are deviated away from each other by 0.75 mm each as shown in FIG. 16, so that a combination of circles whereof the centers are eventually deviated by 1.5 mm defines the outline of the sensing unit. Consequently, the width of the wide portion of the sensing unit is 4 mm and the width of the narrow portion thereof is 1 mm. However, the area of the portions of the coils and the sensing unit overlapped with each other when viewed in the direction orthogonal to the sensing surface (the projecting area of the sensing unit with respect to the coils when viewed in the direction orthogonal to the sensing surface) does not increase proportionally according to the rotation angle of the sensing unit (that is, the rotor). Therefore, the output values of the four areas A in a state of substantially linear other than the projected peaks shown in FIG. 15 obtained via the sensing unit as described above are not linear in a narrow sense.
As described above, since the sensing unit in the related art is simply varied in width in the circumferential direction by simply combining circles in different size, the output voltage directly obtained with respect to the rotation angle of the rotor is sinusoidal signals. It is also conceivable to eventually obtain a desirable linear output using a measure for approximating the sinusoidal signals to linear signals. However, an error may be contained in the course of approximating the sinusoidal signals.
In other words, since the area of the portion of the sensing unit corresponding to the coils does not change linearly with variations in the rotation angle of the rotor, and hence an error is contained in both cases where the output voltage is directly used and where it is approximated to a linear shape, it is difficult to detect the rotation angle with high degree of accuracy with the rotation sensor in the related technology.